1. Field of the Invention
The present invention relates to hydraulic-pneumatic flotation apparatus and more particularly to improvements for increasing efficiency of operation.
2. Description of the Prior Art
Commercially valuable minerals, for example, metal sulfides, apatitic phosphates and the like, are commonly found in nature mixed with relatively large quantities of gangue materials, and as a consequence it is usually necessary to beneficiate the ores in order to concentrate the mineral content thereof. Mixtures of finely divided mineral particles and finely divided gangue particles can be separated and a mineral concentrate obtained therefrom by well known froth flotation techniques. Broadly speaking, froth flotation involves conditioning an aqueous slurry or pulp of the mixture of mineral and gangue particles with one or more flotation reagents which will promote flotation of either the mineral or the gangue constituents of the pulp when the pulp is aerated. The conditioned pulp is aerated by introducing into the pulp a plurality of minute air bubbles which tend to become attached either to the mineral particles or to the gangue particles of the pulp, thereby causing these particles to rise to the surface of the body of pulp and form thereat a float fraction which overflows or is withdrawn from the flotation apparatus.
Typical such flotation apparatus is disclosed in U.S. Pat. No. 3,371,779. In such apparatus, the conditioned pulp is introduced into a flotation compartment containing a relatively quiescent body of an aqueous pulp, and aerated water is introduced into the lower portion of the flotation compartment through orifices formed in the bottom wall. A body of aerated water is provided in a hydraulic compartment disposed directly below the flotation compartment by introducing air and water into the hydraulic compartment in a manner which disperses a multitude of fine air bubbles throughout the water therein. This body of aerated water is in fluid communication with the aqueous pulp in the lower portion of the flotation compartment through the aforementioned orifices. An overflow fraction containing floated particles of the pulp is withdrawn from the top of the flotation compartment and an underflow or non-float fraction containing non-floated particles of the pulp is withdrawn from the pulp in the lower portion of the flotation compartment.
One problem encountered in such prior apparatus is that a portion of the floatable fraction never becomes attached to rising bubbles and therefore passes out of the discharge as part of the tailings. To overcome this, a second apparatus is joined to the first in vertical tandem relation such that the discharge of the upper apparatus feeds into the upper portion of the lower apparatus, the floatable material entrained in this discharge once again becoming subject to the rising currents of bubbles by means of which it may be recovered. This technique for obtaining an increase of the recovered material is an item of some expense, since it requires a duplication of apparatus.
Furthermore, from an operating standpoint, it is undesirable to have flotation cells at two or more levels. An alternative is to have the two cells at the same level and pump from one cell to the other. Pumping complicates the system and adds to operating costs.
In similar apparatus as disclosed in U.S. Pat. No. 2,758,714 instead of injecting aerated water, aerating air without water is introduced directly into a relatively aquiescent body of aqueous pulp by means of air diffusers which are immersed in or are in direct contact with the pulp. Such air diffusers ordinarily include a porous material through the pores or minute perforations of which minute bubbles of aerating air are directly introduced into the aqueous flotation pulp. The size of such pores determines the size of the bubbles and since tiny bubbles are preferred, the pore size must be minimal. As a consequence, a very troublesome problem is encountered because of the tendency of the air diffusers immersed in or in contact with the pulp to become covered with a tenacious coating composed of oily flotation reagents and fine particles of minerals and gangue which clog the minute openings through which air is introduced into the pulp by the air diffusers. Another shortcoming is the tendency of the rising column of air bubbles to become channelized and thereby to unevenly aerate the aqueous pulp.
Such air diffusers may be disposed at different levels within the flotation machine, the air diffusers being in tubular form and spaced horizontally to provide relatively large and unobstructed passages for the aqueous pulp to descend therebetween. In its descent, the pulp is therefore either not retarded at all or only minimally in contrast with the present invention which deliberately retards such descent thereby to prolong the suspension during which bubbles have a greater probability of becoming attached to the floatable particles.
A still further deficiency in certain of the prior art apparatuses resides in the development of circulating currents within the flotation compartment which terminate in the discharge, these currents carrying with them some of the desired, floatable material which is lost as tailings.
This is particularly true when plugging of the aeration tubes becomes uneven.
Other prior art relating to the concentration of minerals by flotation is disclosed in U.S. Pat. Nos. 4,287,054; 2,753,045 and 3,298,519.